SUMMARY Anticoagulants are clinically used to prevent and/or treat thrombotic diseases. Yet, all current anticoagulants are associated with a significant risk of internal bleeding which subsequently limits their effective use in various patient populations. The long-term goal of research in this area is to introduce effective anticoagulants that do not cause bleeding complications so as to be safely used for a wider range of thrombotic patients. Thus, this project aims at developing effective and safe anticoagulants by targeting procoagulant proteases in the intrinsic pathway of coagulation, particularly factor XIa (FXIa) and/or factor XIIa (FXIIa). Clinically used anticoagulants inhibit directly or indirectly thrombin and/or factor Xa of the common pathway of coagulation. This is the reason why they are clinically effective, but it is also the reason why they cause bleeding. The central hypothesis is that inhibiting human FXIa or FXIIa of the intrinsic pathway will result in effective protection against both arterial and venous thromboses without causing bleeding. Inhibitors of FXIa or FXIIa will primarily prevent thrombosis and will leave the hemostatic process essentially intact. In preliminary studies, I identified a sulfonated peptidomimetic that allosterically inhibits FXIa with moderate potency and significant selectivity over thrombin and factor Xa. I also identified a benzamide that orthosterically inhibits FXIIa with high potency and significant selectivity over a panel of related proteases. Not only that but each of these two inhibitors also dose-dependently prolongs the clotting time of human plasma when coagulation is initiated through the intrinsic pathway which further supports their specificity of function. Therefore, the two inhibitors of FXIa and FXIIa exhibit very promising profiles as effective and potentially safe anticoagulants, yet their pharmaceutical and pharmacological properties require further improvement. Thus, I specifically aim to; 1) chemically synthesize an advanced library of sulfonated allosteric inhibitors of FXIa and evaluate their biochemical and biological potential as anticoagulants and 2) chemically synthesize an advanced library of benzamide-based orthosteric inhibitors of FXIIa and evaluate their biochemical and biological potential as anticoagulants. The proposal is innovative because i) it puts forward a novel approach to overcome the limitations of current thrombosis treatment; ii) it exploits a highly integrated, multidisciplinary approach involving synthetic medicinal chemistry, enzyme kinetics, molecular modeling, and testing in human plasmas to investigate the specific aims; and iii) it introduces new technologies with proprietary structural and mechanistic aspects. The project is also significant because it will identify 2-4 potent and selective inhibitors of the intrinsic coagulation pathway for future evaluation in animal models of thrombosis and bleeding.